This invention relates generally to laser guidance systems, and more particularly, to systems which spatially encode a laser beam with position information.
One application of this invention is in the field of missile guidance systems which operate to cause a missile to fly down the center of a transmitted beam, such as a beam-rider guided missile. The advent of the laser has provided a very accurate transmission means for use in these guidance systems.
A typical guidance system incorporates a laser transmitter located at the missile launch point and a receiver located on the missile. The receiver decodes information transmitted by the laser which provides guidance signals to the missile navigation system.
Numerous methods for coding a laser beam have been developed. These include opto-mechanical, electro-optic, acousto-optic and Stark effect modulation. The present invention embodies an improvement in the design of a Stark cell modulation system to produce guidance and control signals.
Most systems attempting to spatially encode a laser beam have utilized polarization coding using electro-optical modulators. These systems require a polarization sensitive receiver which severely limits the field-of-view and generally requires two detectors.
Although systems have been built utilizing electro-optic modulators which need only one detector, the present invention circumvents the polarization problems by using a receiver having a single polarization insensitive detector. Additionally, the need for expensive electro-optical crystals is eliminated.
The electro-optical modulator systems also generally have problems implementing automatic gain control (AGC) functions, especially if the receiver requires two detectors.
Stark effect modulation of laser energy is generally well known and described in numerous texts, articles and patents, including, "Modulation of the 3.394.mu. Ne line by Electro-Optic Gases" by A. Landman, Journal of Applied Physics, Vol. 38 (1967), pages 3668-3675, and Johnson et al, U.S. Pat. No. 3,806,834 entitled "Stark Effect Modulation of CO.sub.2 Laser with NH.sub.2 D". Stark effect modulation has also been used to stabilize laser operation as in U.S. Pat. No. 3,921,099 by R. L. Abrams and T. A. Nussmeier, entitled "Frequency Stabilized Laser".
Modulation of a laser beam by means of the Stark effect utilizes a cell containing a confined gas having a molecular absorption resonance at or near the output frequency of the laser. The resonance can be frequency tuned across the laser bandwidth by generation of a suitable electric field within the cell.
Varying the applied electric field results in changing the resonant frequency of the absorbing gas. Application of an AC modulation signal in addition to the applied DC electric field allows for modulation of the laser beam at the AC modulation frequency.
Previous systems and studies utilizing Stark effect modulators have generally been limited to spectroscopic applications and modulation involving only one modulation frequency.
Thus, it is an object of this invention to provide a means for encoding a laser beam with position information across at least one dimension of the laser beam.
A further object of the invention is to provide an improved position determining system which is adaptable for use in optical beam-rider guidance systems.
Another object of the present invention is to provide an improved system for determining the position of remotely located objects.